Methods utilizing adsorption to separate fluid mixtures are used commercially, for example, on fixed beds of granulated activated carbon or zeolite. It is believed that a majority of the prior art methods suffer from the drawback of being either a cyclic or batch process, in which the sorbent charge is depleted and must be replaced or regenerated after some use. For systems requiring continuous separation, a spare bed must be available for use during regeneration, which often means a duplicate bed and nearly a doubling of capital cost for the separation.
One attempt to solve the problem of fixed beds is to utilize a continuous process such as the Dow Traveling Ion Exchange Bed or a rotating annular chromatograph as described in 30(5) AIChE J 705 (1984). Inherent in any continuous sorptive separation process is the use of a mechanism for moving the sorbent between the sorbing and desorbing (sorbent regeneration) zones. This aspect of sorptive separation was described in U.S. Pat. Nos. 4,548,802 and 4,548,803 which were entitled "Continuous Flow Separation with Moving Boundary Sorption". Math modeling studies of the processes described in these patents reveal it to be approximately competitive with the current fixed bed processes. The modeling indicated that increased separation effectiveness would be achieved to the extent that boundary sorptive surface increased per separator volume.
An effort to achieve greater sorptive surface than possible under the constraints of the moving boundary concept described in said patents led to the new and more comprehensive process described in the applicant's application entitled "Sorptive Contactor", Ser. No. 918,271, now abandoned. The process of that invention abandons the use of sorbent boundary-forming elements.
Standard sorbent materials have vast surface area per unit mass with smallest pore sizes in the nanometer range. This means that the sorbent atoms or molecules can attach sorbate molecules on almost a one-to-one basis. The drawback to use of these materials is that nearly all (99.999% or more depending upon the particle size) of the sorbent surface (sites) are internal. Time is required for the sorbate to get to these sites through the necessarily restrictive pores; desorption is similarly retarded so that removal of large or bulky molecules is not practical. As a consequence, sorptive beds are sized on the basis of providing enough holdup for the sorbate to diffuse to empty internal sites before leaving the bed. This results in relatively large beds or low flow rate compared to the case of equivalent external sorptive surface.
Since an external surface could be quickly saturated, it is practically mandatory that it be regenerated on a high frequency basis to take advantage of its accessibility advantage over porous sorbents; continuous regeneration is optional, allowing the minimum sorber size for a given application.
It is recognized that a means of preparing a mixture of solution of prescribed concentration, such a pharmaceutical preparation, could be effected by the sorptive process. Likewise, contacting fluid (say in the desorption zone) with a permanent catalyst-sorbent, or with reactant carried from the sorption zone--as a sorbate--could provide a means of controlling the rate of a chemical reaction, by controlling the sorbent velocity. Consequently, the sorptive contactor could with appropriate modification be used for any process in which change results from continuous sorptive treatment of a fluid by a solid.
The invention of the copending application is a design for a sorbent system wherein the surface is at or near the geometric (cylindrical) surface of strands which move more or less uniformly in the fluid to minimize transport resistance through the fluid. The sorptive contactor of the copending application disclosed a means of using a moving, multiple strand adsorber to contact fluids in a sequence of zones, the fluid in which could be kept separate and at different conditions of temperature or other environmental parameter. A related device is demonstrated in West German patent No. 3102-280.
In these earlier inventions, a bundle of adsorptive strands are passed through a conventional seal, are separated and drawn through a guide which assures total separation of each of the strands as the strands pass through a sorption vessel. The strands are then gathered into a tight bundle at the other end of the vessel before passing through another seal as it leaves the vessel. The gathering and spreading guides are mechanically cumbersome and can lead to undesirable abrasion of the sorbent surface. Thus, it can be seen that the strands coated with sorbate in the sorption zone are tightened into a bundle, passed through a mechanical seal to leave the sorption vessel, passed through an additional mechanical seal to enter the desorption vessel, and then passed through a spreading guide before the strands contact the desired fluid in the desorption vessel.
The surface of the strands of the sorptive contactor is coated with a sorbate layer, which may be very thin, possibly only one molecule of sorbed material at each surface site. A difference in strand velocity between each of the multiple strands would likely have enough frictional force to disturb the adsorbate or the sorbing surface. Currently available treatments to polymeric surfaces to make them selectively sorptive to biological molecules are relatively expensive and the treated surface fragile. This fragility is compatible with stationary gel type separation methods but not of methods where solid shearing or slip occurs at the treated surface, as can occur in the sorptive contactors which bundle strands together as they pass between the sorption and desorption zones.
It is therefore a principal object of the present invention to provide an improved coordinated sorptive strand contactor.
A further object of the invention is to provide a sorptive contactor which reduces the abrasion of the contactor strands between the sorption zone and desorption zones.
Yet another object is to provide a sorptive contactor which achieves great, external and readily accessible surface in the sorption and desorption zones.
Still another object of the present invention is to provide a sorptive contactor wherein a series of separated strands are exposed to liquid mixtures in the sorption and desorption zones.
Yet another object is to provide a sorptive contactor having separated strands formed of a single spiral endless loop of sorbent material.
Yet a further object of the present invention is to provide a sorptive contactor with separated strands which pass from the sorption zone to the desorption zone without abrasion.
These and other objects will be apparent to those skilled in the art.